Integrated circuit chip packages are typically formed by mounting an integrated circuit chip on a lead frame and coupling these two elements to form a package. The integrated circuit chip and lead frame may be encapsulated. Typically, the chip includes a number of bond pads which may be positioned about a perimeter of the chip according to a predetermined spacing between the bond pads. The lead frame typically includes a number of leads about a perimeter thereof. One type of lead frame, for example, has a generally rectangular shape with each side of the rectangle having a number of leads. The leads may each have a relatively narrow elongated shape.
A lead frame may be said to have an X direction and a Y direction. The X direction is perpendicular to one pair of opposing sides and the Y direction is perpendicular to the other pair of opposing sides. Typically, each lead has a relatively narrow elongated shape defining a lead axis. The lead axis for any given lead frame may extend in the X or Y direction, or be offset by an angle from either the X or Y direction. The angle of offset may vary from lead to lead. Moving from the center of a given side of the lead frame toward the corner leads, the lead axes may be angularly offset from perpendicular by increasing amounts. Also, a bonding path is defined by the direction from a bond pad to a corresponding lead. The bonding wire may extend along the bonding path. For any given set of corresponding bond pads and leads, the bonding path may extend in the X or Y direction, or be offset by an angle from either the X or Y direction.
In order to electrically couple the integrated circuit chip to the leads of the lead frame, a wire bonding technique is often used. A wire bonding machine may have a spool of bonding wire mounted on the machine. The bonding wire may be threaded through a capillary which is mounted to a horn of the wire bonding machine. The horn may be manipulated to move the capillary both vertically and horizontally. Typically, the wire bonding machine includes a device for heating or applying a spark to an end of the bonding wire which protrudes from an exit end of the capillary. The molten wire may form the shape of a ball which is placed on a target bond pad by manipulating the horn to move the capillary.
After this bond pad bond is created, a sufficient amount of bonding wire is released to allow the capillary to be moved to a location near an inner end of a target lead of the lead frame. The capillary is manipulated to connect the bonding wire to the inner end of the target lead and cut off the bonding wire so that the bonding wire protruding from the exit end of the capillary is now free to form a new wire bond between a new target bond pad and target lead. Any type of suitable bond may be made at either the bond pad or the lead, including ball bonds, stitch bonds and wedge bonds. A ball bond may be used, for example, at the bond pad. A stitch bond may be used, for example, at the lead. To complement the bonding process the package may be heated. Also, ultrasonic energy may be applied.
Problems in wire bonding techniques arise in part from the desire to increase the number of leads in a given package and to make integrated circuit chip packages smaller and smaller. This may require that the bonding pads located on the chip be made smaller and be spaced closer together. The same can be said for the leads on a lead frame.
The exit end of a wire bonding capillary is often referred to as the capillary face. Previous capillaries have had a circular face. A disadvantage of having a capillary with a circular face is that the spacing between bonds is limited. After a bond is made at a particular bond pad, for example, if the adjacent bond pad is too close then the capillary face may strike the ball bond which has been made at the first bond pad during the process of making a bond on the adjacent bond pad. One method for solving this shortcoming is to use a wire bonding capillary with a non-circular face. This type of approach is shown, for example, in U.S. Pat. No. 5,544,804 issued to Test et al., which is hereby incorporated by reference for all purposes. The Test et al. patent shows a BowTI.TM. capillary having a non-circular face. The face of a BowTI.TM. capillary may have a shape which includes a pair of opposed convex sides joining a pair of opposed concave sides. The BowTI.TM. capillary may be generally described as having a longitudinal axis extending across the midpoints of the convex sides and through the center of the BowTI.TM.. The BowTI.TM. capillary allows ball bonds, for example, to be made closer to one another than with a circular capillary face. This can be accomplished because the concave sides avoid striking adjacent bonds. The BowTI.TM. capillary can also make other types of bonds including stitch bonds.
A need arising from the use of capillaries having non-circular faces is precise alignment of the longitudinal axis of the capillary face along either the X or Y direction of the lead frame, or along the longitudinal axis of a target lead, or along a given bonding path as necessary. Precise alignment of non-circular capillaries is especially difficult due to the relatively small size of a typical capillary face (e.g., 4-8 mils). Improper alignment of the capillary, particularly in view of the decreasing size of integrated circuit chip packages, can lead to defective wire bonds during the manufacturing process. This can result from many factors including improper positioning of the capillary face over the bond pad, the lead or both. Defective wire bonding can also occur when improper alignment causes the capillary to strike and/or damage an existing bond during the formation of a subsequent bond. Other problems, shortcomings and disadvantages of known capillaries and wire bonding techniques exist.